Digital television signals are defined in various standards which we will refer to as the DVB standards. This includes DVB, DVB-T2 and newer standards such as DVB-NGH. The DVB-NGH standard DVB Document A160 “Next Generation broadcasting system to Handheld physical layer specification” relates particularly to handheld terminals for receiving digital terrestrial and hybrid (combination of terrestrial with satellite transmissions).
OFDM-MIMO systems are described more generally in our own earlier European Patent EP1821481 which describes an OFDM-MIMO system in the form of a DVB-T system with modifications to the standard DVB-T transmission signal that allow the receiver to have knowledge of the resulting 2-by-2 matrix channel. Our own earlier United Kingdom Patent GB2449858 describes further improvements to OFDM using cyclic delay diversity in which a cyclic delay may be applied to OFDM carriers in a manner such that the apparent delay varies as a function of frequency across the spectrum. OFDM and MIMO are known to the skilled person, particularly in the context of digital television, but will be described briefly here for ease of reference later.
OFDM
COFDM (coded orthogonal frequency-division multiplex) techniques can be used for the transmission of any digital information. In brief, in COFDM, data is divided between a large number of (typically more than a thousand) closely-spaced carriers. This explains the ‘Frequency Division Multiplex’ part of the name COFDM. Only a small amount of the data is carried on each carrier, and this significantly reduces the influence of intersymbol interference.
The distribution of the data over the many carriers means that selective fading will cause some bits to be received in error while others are received correctly. By using an error-correcting code, which adds extra data bits at the transmitter, it is possible to correct many or all of the bits which were incorrectly received. The information carried by one of the degraded carriers is corrected because other information, which is related to it by the error correction code, is transmitted in a different part of the multiplex (and, it is hoped, would not suffer the same deep fade). This explains the ‘Coded’ part of the name COFDM.
The ‘Orthogonal’ part of the COFDM name indicates that there is a precise mathematical relationship between the frequencies of the carriers in the system.
The receiver acts as a bank of demodulators, translating each carrier down to dc, the resulting signal then being integrated over a symbol period to recover the raw data. If the other carriers all beat down to frequencies which, in the time domain, have a whole number of cycles in the symbol period (t), then the integration process results in zero contribution from all these other carriers. Thus, the carriers are linearly independent (i.e. orthogonal) if the carrier spacing is a multiple of l/t.
The process of creating an OFDM signal may be summarised by the following steps.
A serial digital signal comprising a bit stream is converted into a plurality of parallel bit streams. Using a chosen modulation scheme, such as Binary Phase-Shift Keying (BPSK) or Quadrature Amplitude Modulation (QAM) the parallel bit streams are mapped to a plurality of subcarriers. BPSK modulates one bit per carrier, in 4-QAM there 4 are carrier states of equal magnitude each separated by 90 degrees and so this modulation scheme can carry 2 bits on each carrier. With higher order levels of QAM more bits per carrier may be modulated.
The plurality of modulated carriers are chosen to have a frequency spacing that is the inverse of the active symbol period over which the receiver will examine the signal. It is the choice of carrier spacing in relation to the active symbol period that ensures the orthogonality of the carriers. At the receiver the demodulator for one carrier does not “see” the modulation of others.
The original input bit streams which are now mapped onto carriers can be thought of as frequency coefficients. Performing a Fourier transform on frequency coefficients transforms a frequency domain signal to a time domain signal (a signal varying in amplitude with time). Accordingly, the bit stream data representing the modulation onto groups of modulated carriers is fed to an Inverse Fast Fourier Transform (IFFT) block which transforms the data into a time domain modulated signal comprising symbols, with groups of symbols arranged into frames with appropriate guard intervals between the symbols. Each symbol results from one set of modulated sub-carriers.
The “frequency” of symbols within an OFDM signal is typically described as that of either the lowest carrier or the centre carrier in the set of carriers used. In reality, of course, the OFDM signal is a signal varying in amplitude with time that is formed from the plurality of carriers, as described above. It is useful, though, to describe the “frequency” of a symbol in this way, particularly in the context of DVB signals.
MIMO
Methods of delivering digital wireless television have been proposed which use Multiple-Input Multiple-Output (MIMO) techniques to allow dual transmission streams to be transmitted. In a typical basic system there are two transmit antennas and two receive antennas, with associated transmitters and receivers. Such a system can deliver up to twice the throughput of conventional Single-Input Single-Output (SISO) systems, whilst requiring no additional spectrum. More generally MIMO refers to a radio link employing at least two (two or more) transmitters and two or more receivers.